June 29, 2014
By Tom Burns
Here is how we left our galaxy, the Milky Way, last time. In 1755, German philosopher Immanuel Kant theorized that the galaxy was a flattened disk made up of stars.
In the meantime, astronomers had discovered another component to the universe. Telescopes showed “nebulae,” fuzzy patches that appeared to live among the stars of the Milky Way. Some were clearly misshapen clouds. Others resolved into stars and were surely part of the Milky Way. Others clearly had egg and disk shapes. In the latter case, Kant speculated that those nebulae might actually be complete and unto themselves. They might be other galaxies, or “island universes” as they came to be called.
Galileo’s first use of the telescope in 1609 had proven that the Milky Way was made up of stars. The next step was to prove that the Milky Way was a flattened disk.
It took another giant leap forward in telescope technology — and obsessive soul who knew how to exploit it - to produce the proof of Kant’s assertion.
From the telescope’s very beginning in 1608, astronomers had noticed that lens-based refracting telescopes had their limitations. They produced fringes of false color around astronomical objects, and they were difficult to make in high magnifications and sizes. Also, the single lens that gathered the light from stars tended to produce distorted images because of an inherent flaw called spherical aberration.
The partial solution was to make the telescopes very long — so long that they became increasingly unwieldy to use. Also, lenses were difficult to produce in the large diameters necessary to increase the visibility of very faint objects.
In 1668, British scientist Isaac Newton had come up with a solution to all of those problems. Instead of bending the light with lenses, he proposed to collect the light with mirrors. His tiny 2-inch-diameter astronomical telescope mirror spawned a revolution that was not to bear fruit until more than 100 years later.
The problem was that the mirrors were made of speculum metal, a mixture of copper and tin that can be polished to a very reflective surface. Speculum metal can be cast into a rough approximation of the required concave surface, but the process took many hours of grinding and polishing to produce a correct curve with maximum reflectivity. The process was grueling, time consuming, and physically demanding.
Luckily for astronomy, English astronomer, mathematician, musician, composer, and workaholic William Herschel appeared on the scene. Starting in the early 1700’s, he began grinding and polishing telescope mirrors of increasing size.
He did this work at his own residence, known as Observatory House, in Slough, England. He worked for 60-hour stretches without rest. When he was finally done with a mirror, he endured, time after time, the inevitable tarnishing of the metal. After six months of use, every mirror had to be meticulously polished again. Still, he soldiered on. The telescopes grew larger and larger, culminating in a mirror that was the largest in history at a whopping 49.5 inches in diameter.
It’s a wonder that he had time to use the telescope, but even his smaller mirrors gathered much more light than the lens-based telescopes of his day. Among his discoveries was the planet Uranus, the first to be added to the planet pantheon since prehistoric times.
One of Herschel’s projects was to measure the approximate distance to as many stars as possible. His meticulous, if somewhat inaccurate, efforts allowed him to estimate the distance to several hundred stars.
By 1775 he was ready to attempt to determine the shape an size of the Milky Way. He came up with Kant’s depiction, more or less, with strange extensions and appendices.
He concluded that most of the stars of the stars of our galaxy are located in a circular band around the sky, suggesting that we are located in a disk of stars, with the plane of the disk aligned with the hazy Milky Way. His measurements suggested that the thickness of the disk was about one-tenth its diameter.
His work was not without its limitations. Since he had no way of measuring the actual distance to any given star, he could not gauge the size of the disk. Also, he assumed that the sun was near the center.
His observations ultimately failed because Herschel made several false assumptions. The most incorrect was that all stars shine with the same, intrinsic brightness. Another was that stars are arranged uniformly throughout the Milky Way.
He also assumed that the space between stars is essentially empty. We now know that dust between the stars absorbs some of the light before it gets to us. He also assumed that he could see to the edge of the Milky Way. Unfortunately, that same interstellar dust is bound to absorb all the light from very distant stars.
Still, the results were startling. Here at last was concrete evidence for the philosophical speculations of Kant so long before.
These days, we can get all the proof we need simply by looking up at the glorious Milky Way on some warm, clear, moonless summer night.
Tom Burns is the director of Perkins Observatory. He can be reached at email@example.com.